Such a method is intended to enable differences in the injection quantity between the cylinders to be detected and adjusted if necessary. It is precisely in order to achieve small injection quantities, particularly of advance injection quantities in the region of a few milligrams, that determining the actuator energy is crucial for complying with exhaust emissions. When injectors are equalized by means of the actuator signal, stationary operating points lasting several seconds (3 to 4 seconds) must be observed, depending on the injection pressure, to enable the actuator energy of the main injection to be determined. During active control, the time between the triggering of the injector and the actuator signal of all injectors is set to a value stored in the engine characteristics map. Since fairly large injection quantities are required for reliable evaluation of an actuator signal, only the main injection may be included in order to determine the actuator energy. This energy for the main injection is used as the basis for the energy for the advance injection and after-injection.
However, it has been found that the advance injection generates local variations in pressure in the incoming injector line. These local variations in pressure cannot be detected by the pressure gage in the rail, so that the pressure appears constant to the control system. These local pressure variations have a significant effect on the opening behavior of the injector and thus on all subsequent injections. The actuator signal of the main injection therefore displays a clear dependence on the separation angle between the advance injection and after-injection. If the main injection is effected with “maximum pressure”, for example, the needle of this injector will be opened more quickly because of the locally higher pressure. As a result, the actuator signal is generated at an earlier point in time. This causes the control system to reduce the actuator energy for the corresponding injector in order to set the time between the start of the control and the generation of the actuator signal to the value set in the engine characteristics map. Even if the energy setting for the main injection is correct, the advance injection depending on it still receives too little energy.